(1) Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method of manufacturing the same, and more particularly, to a solid electrolytic capacitor having a two-layer structure of a electrically conducting polymer as a solid electrolyte layer.
(2) Description of the Related Art
A solid electrolytic capacitor comprises an etched foil or a sintered body of powder of a valve metal such as aluminum or tungsten as the anode body, and an oxide film formed on the surface of the anode body serving as a dielectric body. The capacitor further comprises a negative electrode layer formed on the oxide film consisting of a solid electrolytic layer, a graphite layer formed thereon, and a metallic layer such as a silver paste layer.
In recent years, a conductive polymer is used as the solid electrolyte instead of using a conventional manganese dioxide. Since a conductive polymer has a higher electrical conductivity compared with the manganese dioxide, it is expected to obtain a solid electrolytic capacitor having an excellent frequency characteristic with small loss component even at high frequencies.
The conductive polymer as a solid electrolyte can be formed by using either one of methods of vapor phase polymerization, chemical polymerization or electrolytic polymerization.
The vapor phase polymerization is a method in which the anode body is immersed in a solution containing an oxidant and a dopant, then exposing the anode body to the monomer vapor of the conductive polymer to induce polymerization.
The chemical polymerization is a method in which polymerization is effected by alternately immersing the anode body in a solution containing an oxidant and a dopant and a monomer solution of a conductive polymer, or by preparing a reaction solution at a low temperature in advance, immersing the anode body in the reaction solution, then effecting polymerization in the process of raising the temperature.
The electrical polymerization is a method in which the anode body is immersed in an electrolytic solution containing the monomer of a conductive polymer dissolved therein, and effecting polymerization by applying a voltage.
Each of these polymerization methods has its own feature. Namely, the vapor polymerization produces a layer which shows an excellent adhesion to the dielectric body, but the layer produced is coarse such that its adhesion to the graphite layer and the silver paste layer is poor when left as it is and its characteristics are unstable.
The chemical polymerization also produces a layer with an excellent adhesion to the dielectric body, but the compactness of the produced layer is inferior to that obtained by the electrolytic polymerization. The electrolytic polymerization can form a compact layer, but its adhesion to the dielectric body is poor, and the method requires an underlying layer which serves as an electrode in applying a voltage.
For these reasons, two-layer structure of conductive polymer as the solid electrolytic layer is suggested. For example, Japanese Unexamined Patent Application (Kokai) No. 64-21913 (1989) shows a method in which a solid electrolytic layer consisting of a second conductive polymer compound obtained by the electrolytic polymerization is formed on a solid electrolytic layer consisting of a first conductive polymer compound obtained by the vapor phase polymerization. Further, another Japanese Unexamined Patent Application (Kokai) No. 3-46215 (1991) discloses a method in which a conductive polymer film obtained by the electrolytic polymerization is formed after the formation of a conductive polymer film obtained by the chemical polymerization is formed, in order to obtain a solid electrolytic layer.
In these two-layer structure, since the solid electrolytic layer is constituted exclusively of a soft conductive polymer, the strength for protecting the oxide film against an external stress is not sufficiently high compared with manganese dioxide.
Because of this, when an external mechanical stress is applied to the outside of the capacitor, the oxide film tends to be damaged, and the leakage current characteristic of the capacitor is deteriorated. In particular, since a large stress is generated at the time of the external packaging with resin during the manufacturing processes, the leakage current increases after this process.